1. Field of the Invention
This invention relates generally to a method of fabricating polymer nanopillars, and in particular to a method of fabricating polymer nanopillars using an anodic aluminum oxide (AAO) membrane and an imprint process.
2. Description of Related Art
Coating approaches including spray coating, surface etching and electrospinning provide nanostructural patterning. These coatings provide much higher surface area for analyte absorption and desorption than solid film coating, and reduce diffusion length due to nanoscale polymer thickness thereby improving the sensitivity significantly. However, the existing nanostructural coating approaches are not able to accurately control the coating parameters such as porosity, thickness, and geometry and especially to provide uniform nanostructure patterns with polymer materials. On the other hand they are expensive and need sophisticated equipment and complex procedure. Especially the random nanopatterns cannot guaranty a repeatable performance for applications such as sensors even though it was fabricated under the same procedures.
Nanoimprint lithography (NIL) was first developed by Chouet al., “Imprint Lithography with 25-NANOMETER Resolution”, Science 1996, Vol. 272, No. 5258, pp. 85-87. Although it has a history of just 15 years, NIL has rapidly developed because of providing repeatable and low cost approach for nanostructural patterning. The NIL needs two basic parts: a master mold with nanostructure pattern and a substrate coated with imprint resist (thermal or UV resist) coating. By controlling the temperature and pressure between master mold and the substrate, the nanostructure pattern can be transferred from the mold to the resist layer on the substrate. Comparing to the traditional nanoscale fabrication methods such as electrospinning process (see S. Liu et al., “Dynamic Chemical Vapor Sensoring With Nanofibers Film Based Surface Acoustic Waves Sensors, Sensors and Actuators A.”, Vol. 167, Iss. I, pp. 8-13.), the nanoimprint coating approach is much easier for control of thickness, pattern geometry and locating.
U.S. Patent Application Publication No. US2010/0145491 dated Jun. 10, 2010 discloses a method and apparatus for the controlled fabrication of micro and nanoscale structures as by thermocapillary, lithography. The growth and evolution of 2D and 3D structures are controllably stimulating from a thin film mass transfer process such that complex devices can be designed and fabricated having engineered features of different height and separation distances in a single or few process steps. However, the length of the nanopillars is limited to a few hundreds of nanometers whereas in the present invention, the nanopillars can be fabricated up to a few microns. Also, the length and size of these nanopillars are limited by the type of polymers and the surface tension of the polymers.
U.S. Patent Application Publication No. US2011/0008484 dated Jun. 13, 2011 discloses a method for forming a nanoimprint mold, comprising the steps of filing nanoholds formed in a substrate with a material to form nanopillars on the substrate, performing at least a first partial oxidation of the nanopillars and then removing at least a portion of the oxidized material, depositing a hard substance on the nanopillars to begin forming the nanoimprint mold on the nanopillars, depositing a second material on the hard substrate, and removing the substrate from a molding surface of the nanoimprint mold. However, the holes on the mold usually are shallow and then the nanopillars are short in size (less than 400 nm), but the present invention can form both short and long nanopillars/wires depending on pressure such as a few micrometers. Also, the density of the nanopillars formed by this method is low.